Heme oxygenase (HO) catalyzes the first and rate limiting step in the degradation of heme to yield equimolar quantities of biliverdin IXa, carbon monoxide (CO), and iron (Choi, et al., Am. J. Respir. Cell Mol. Biol. 15: 9-19; and Maines, Annu. Rev. Pharmacol. Toxicol. 37: 517-554). Three isoforms of HO exist; HO-1 is highly inducible while HO-2 and HO-3 are constitutively expressed (Choi, et al., supra, Maines, supra and McCoubrey, et al., E. J. Bioch. 247: 725-732). Although heme is the major substrate of HO-1, a variety of non-heme agents including heavy metals, cytokines, hormones, endotoxin and heat shock are also strong inducers of HO-1 expression (Choi, et al., supra, Maines, supra and Tenhunen, et al., J. Lab. Clin. Med. 75: 410-421). This diversity of HO-1 inducers has provided further support for the speculation that HO-1, besides its role in heme degradation, may also play a vital function in maintaining cellular homeostasis. Furthermore, HO-1 is highly induced by a variety of agents causing oxidative stress including hydrogen peroxide, glutathione depletors, UV irradiation, endotoxin and hyperoxia (Choi, et al., supra, Maines, supra and Keyse, et al., Proc. Natl. Acad. Sci. USA. 86: 99-103). One interpretation of this finding is that HO-1 can serve as a key biological molecule in the adaptation and/or defense against oxidative stress (Choi, et al., supra, Lee, et al., Proc Natl Acad Sci USA 93: 10393-10398; Otterbein, et al., Am. J. J. Respir. Cell Mol. Biol. 13: 595-601; Poss, et al., Proc. Natl. Acad. Sci. USA. 94: 10925-10930; Vile, et al., Proc. Natl. Acad. Sci. 91: 2607-2610; Abraham, et al., Proc. Natl. Acad. Sci. USA. 92: 6798-6802; and Vile and Tyrrell, J. Biol. Chem. 268: 14678-14681. Our laboratory and others have shown that induction of endogenous HO-1 provides protection both in vivo and in vitro against oxidative stress associated with hyperoxia and lipopolysaccharide-induced tissue injury (Lee, et al., supra, Otterbein, et al., supra and Taylor, et al., Am. J. Physiol. 18: L582-L591). We have also shown that exogenous administration of HO-1 via gene transfer can provide protection against oxidant tissue injury and elicit tolerance to hyperoxic stress (Otterbein, et al., Am. J. Resp. Crit. Care Med. 157: A565 (Abstr)).
Carbon monoxide (CO) is a gaseous molecule with known toxicity and lethality to living organisms (Haldane, Biochem. J. 21: 1068-1075; and Chance, et al., 1970, Ann. NY Acad Sci. 174: 193-204.). However, against this known paradigm of CO toxicity, there has been renewed interest in recent years in CO behaving as a regulatory molecule in cellular and biological processes based on several key observations. Mammalian cells have the ability to generate endogenous CO primarily through the catalysis of heme by the enzyme heme oxygenase (HO) (Choi, et al., supra and Maines, supra). The total cellular production of CO is generated primarily via heme degradation by HO (Marilena, Biochem. Mol. Med. 61: 136-142 and Verma, et al., 1993 Science 259: 381-384). Moreover, CO akin to the gaseous molecule nitric oxide, plays important roles in mediating neuronal transmission (Verma, et al., supra and Xhuo, et al., Science 260: 1946-1950) and in the regulation of vasomotor tone (Morita, and Kourembanas, 1995, J. Clin. Invest. 96: 2676-2682.; Morita, et al., 1995 Proc. Natl. Acad. Sci. USA 92:-1479; and Goda, et al., 1998, J. Clin. Inv. 101: 604-12). There is no data in the literature substantiating a protective role for CO in vivo against oxidative stress.
Septic shock and sepsis syndrome, resulting from excessive stimulation of immune cells, particularly monocytes and macrophages, remains one of the leading causes of death in hospitalized patients. Parillo, et al., Ann. Intern. Med. 113, 991-992 (1992). The pathophysiological alterations observed in sepsis are often not due to the infectious organism itself, but rather to the uncontrolled production of pro-inflammatory cytokines and chemokines including TNF-α, IL-1, and MIP-1 that leads to leukocyte recruitment, capillary leak and ultimately participates in the lethality of sepsis. Beutler, et al., 232, 977-980 (1986); Netea, et al., Immunology 94, 340-344 (1998); and Wolpe, et al., J. Exp. Med. 167, 570-581 (1988). Lipopolysaccharide (LPS), a constituent of the gram negative bacterial cell wall, is the leading cause of sepsis, and when administered experimentally to macrophages or mice, mimics the same inflammatory response. Following LPS administration, there is a rapid but transient increase in these pro-inflammatory mediators which are subsequently down-modulated by a battery of anti-inflammatory cytokines including interleukin-10 (IL-10) and interleukin-4 (IL-4), which inhibit the synthesis of the pro-inflammatory cytokines and chemokines. J. Exp. Med. 177, 1205-1208 (1993). LPS initially binds to the CD14 and toll-like receptor (TLR) 2 (or 4) at the cell surface, [Yang, et al., Nature. 395: 284-288 (1998) and Chow, et al., J. Biol. Chem. 274: 10689-10692 (1999)] and has then been shown to activate the mitogen activated protein (MAP) kinase pathways including p38, p42/p44 ERK and JNK (MAP) kinases. Liu, et al., J. Immunol. 153, 2642-2652 (1994); Hambleton, et al., Proc. Natl. Acad. Sci. USA. 93, 2274-2778 (1996); Han, et al., J. Biol. Chem. 268, 25009-25014 (1993); Han, et al., Science 265, 808-811 (1994); Sanghera, et al., J. Immunol. 156, 4457-4465 (1996), and Raingeaud, et al., J. Biol. Chem. 270, 7420-7426 (1995). The relationship between the activation of these signaling molecules, downstream cytokine expression, and physiologic function represents an active line of investigation.
The United States Government has provided support for research which led to the present invention under one or more of NIH grant numbers HL60234, A142365 and HL55330. Consequently, the government retains certain rights in the invention.